Continuity mistake: Look at the forehead of the hotel manager. Everytime he raises his eyebrows, you can see his prosthetic forehead piece.
Revealing mistake: In one of the scenes in the ballroom you can see that the thumb claw of the head witch is put on top of her real thumb.
Continuity mistake: When the first witch turns into a mouse, the way her empty dress lies on the table changes between the first and second shot, in a mere frame. Check the plate on the left, for example: it swaps from uncovered to covered by the dress. Also, the first shot shows more lace by the edge of the table, while the second shot shows much more silky fabric.
Continuity mistake: When Grandma shows her bag with the mice inside to Bruno's mom, her grip on the bag changes between shots: sometimes she is grabbing the upper handle where the yellow threads are; others, the wooden part slightly below the threads; and yet in other shots she is grabbing the lower wooden part. This changes back and forth. Her right thumb also appears and disappears randomly.